The Earth itself comprises a biosphere in which microorganisms, plants, and animals, including humans, exist in a more or less steady state, wherein matter is a finite resource which is continually recycled. There is continual energy input in the form of solar radiation. The quantity of matter gained or lost to space outside the Earth's atmosphere is minute. Thus, the Earth is a closed ecological system or biosphere. Although the Earth recycles matter continually between the soil, oceans, atmosphere, biomass, and the like, in a nearly steady state, there appear to be a drift of conditions as indicated by increasing carbon dioxide concentration in the atmosphere, the well-known "ozone hole," and other effects which may not have been detected as yet.
It is desirable to provide a microcosm of the biosphere known as Earth for study of the interaction of components, and development of techniques for influencing our environment. Such experiments are difficult at best in the open system provided on Earth, where matter is exchanged between the Earth's environment and the experiment itself. It is, therefore, desirable to provide a system that is completely enclosed so that no matter is exchanged with the Earth's environment. It is desirable to have humans within this miniaturized biosphere to provide control, and conduct scientific research within a closed system where conditions may be varied as desired.
Being a closed system having humans within the system requires that they be provided with a balanced diet for long-term health and that closed cycles be established for carbon, oxygen, nitrogen, other nutrients, water, and the like, so that the closed system may remain in a more or less steady state for the long periods of time required for conducting meaningful scientific research.
There is, therefore, being established near Oracle, Arizona, a completely closed ecological system referred to as Biosphere 2. The system completely encloses a hectare of land and 142,000 cubic meters of space isolated from the Earth's environment by an impermeable skin so that no matter is transferred. The portion of the skin above the ground is transparent for receiving solar radiation. Electrical energy is provided to the closed system, and heat may be transferred to or from the system as required. Thus, the Biosphere 2 closed ecological system is closed as to matter, but open as to energy. For meaningful research, it also remains open for information transfer.
The Earth's barometric pressure varies according to a large number of different factors, but remains consistent within a limited range because the Earth's gravitational pull and the mass of the atmosphere remain consistent. The effects of local changes in temperature and atmosphere density are quickly dissipated to other local areas.
Biosphere 2 is a relatively small completely airtight enclosure which, therefore, has important differences from the biosphere of Earth and from a conventional building enclosure. In a conventional building, when the air inside is heated or cooled or has a change in humidity, the resulting expansion or contraction of that atmosphere will leak out or draw outside air into the building. In such a case, no important pressure difference is created between the inside and the outside of the building because the leakage slowly equalizes the pressures. Because Biosphere 2 is a closed structure, there is no leakage. Therefore, unless the structure were built to be much stronger than otherwise necessary, the resulting pressure differences could burst or crush the structure.
Biosphere 2 is designed to prevent these dangerous forces by incorporating pressure balancing "lungs". The lungs are a part of the enclosure that is able to change its volume in response to changes in temperature and/or humidity inside Biosphere 2 or barometric pressure outside Biosphere 2. The weight of the movable portion of the lungs provides a controlled positive pressure inside the Biosphere 2 to prevent inward leaks. Two such lungs provide the desired variable volume in this case, although one or more lungs of adequate total volume could in principle suffice.